Aerodynamics: How Wings Work

When people first tried to fly, probably thousands of years ago, they made wings and flapped them like a bird. An aircraft based on this principle is known as an ornithopter. Nobody has yet made a practical ornithopter; rather, successful aviation has been developed through a knowledge and understanding of aerodynamics.

The basic forces acting on an aircraft in flight.

 

1. Sir George Cayley's fixed wing

In 1799 a Yorkshire man. Sir George Cayley, proposed that lift and propulsion should be separated. He suggested that (leaving aside the question of propulsion, which would require oars or propellers) lift should be provided by a fixed wing. He imagined this as an inclined plane, a flat sheet set at an angle to the oncoming air. This angle became known as the angle of attack. Cayley also saw that, for stability, a flying machine would need a tail.

Thus, Cayley invented the aeroplane. Of course, it is very difficult to make a thin sheet that retains its strength if bent. Early aeroplane designers realised that they had to add a strong frame or pylon above and below the fuselage (body) and join its ends to the wing with bracing wires. By 1809. Cayley had successfully tested both models and full-sized machines.

 

2. The cambered wing

In fact, a flat inclined plane is very inefficient as a lifting surface. By 1850. most of the many drawings for suggested aircraft showed wings with a slight camber (curvature), convex on top and concave underneath. These could meet the air with a zero angle of attack, so that the air would not tend to break away from the upper surface, but would flow smoothly over the wing. In doing so, the air would be accelerated downwards, the reaction providing lift.

 

3. Alphonse Penaud and his Planophore

Still no aviator succeeded in actually flying but, in 1871, a Frenchman, Alphonse Penaud, made a model that worked. Called a Planophore, it had cambered monoplane wings and a propeller driven by twisted elastic. Thousands were later to be sold as toys.

 

4. The first true aerofoils

The first would-be aviator to approach the problem scientifically was Englishman Horatio Phillips. He virtually invented the science of aerodynamics. In 1884. he took out his first patent for a cambered wing and then tested dozens of different aerofoil sections. These were not just cambered sheets, but aerofoils (wings) that had a rounded leading-edge, were thicker in the middle, and had a sharp trailing-edge.

These illustrations show the aerofoil sections included in Phillips' patent of 1884.

 

5. Classic biplane configuration

By 1930, many thousands of aircraft had been built. Nearly all were biplanes. Because the upper and lower wings made a strong but light box, joined by struts and braced with wires, the wings themselves could be quite thin. The Curtiss Jenny (illustrated) demonstrated a typical biplane configuration. The biplane made sense structurally, but offered comparatively high drag and was ultimately to be abandoned as construction techniques improved and aircraft of higher performance were developed.

 

6. The aerofoil at work

Concentration on the biplane tended to make designers forget how wings with a proper aerofoil section (as distinct from a flat inclined plane) actually worked. The key factor is that the air molecules have to travel further to cross the cambered upper surface than to go across the flat, or almost flat, underside. This means the air is speeded up across the top. To plot airflow graphically, aerodynamicists gave the name 'streamlines' to the paths taken by individual air molecules. An undisturbed airflow is represented by a number of parallel streamlines, as shown at the left of the illiustration. The speed of the air increases as it flows across the top of the wing, and the streamlines get closer together

 

7. Bernoulli's experiments

Almost 300 years ago, a scientist named Bernoulli discovered that, for a perfect gas (air is not quite 'perfect' because it is viscous, ie the molecules stick together), at every point along a streamline the total energy is always constant. The total energy is made up of potential energy, pressure energy and kinetic energy (energy due to speed) Surprising results came from Bernoulli's experiments.

Manometers showing the air pressureat different points in the tube

Imagine air flowing through a tube which narrows to a point of minimum cross-section, called the throat, and then opens out again. Such a tube is called a venturi. What happens to the pressure as the air flows through this venturi! The obvious answer might be that it increases to a maximum at the throat and then returns to normal. This is what might occur with a crowd of people trying to escape through a passage that gets increasingly narrower. In fact, as Bernoulii demonstrated, exactly the opposite happens. As the air flows through the tube, its velocity (indicated on the circular dials) increases to a maximum at the throat and then returns to the original value. Although it is hard to believe, to keep the total energy constant, the pressure falls to a minimum at the throat, and then returns to the original value. Illustrated top right is a venturi beneath an aircraft cockpit, its throat is connected by a pipe to the aircraft's gyro instruments. As the aircraft flies, the venturi sucks air out of the instruments. Air rushes in to replace the flow sucked out, and is squirted on to buckets on the edges of the gyro wheels which power the instruments, turning them at high speed.

 

8. Resultant lift

Now magine a venturi cut open and unrolled so that it forms the top surface of a wing. As air flows across it, the pressure falls, in effect sucking the wing upwards. If it still seems impossible to believe that the pressure falls as the air rushes over the wing, try a simple experiment wth a sheer of paper. Details of the original experiment illustrated below were first published in 1910.

The variation in pressure as the air flows past the wing causes forces to act all round it. These can be drawn as vectors (arrows showing strength and direction) All of these forces added together produce the resultant lift, which acts through the centre of pressure.

Cross-section showing forces acting on an aerofoil

 

This page was borrowed from the World Aircraft Information Files, which is produced by Areospace Publishing Ltd. and published by Bright Star Publishing plc. www.airpower.co.uk